<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .container {
      height: 100vh;
    }
    .menu {
      width: 200px;
      float: left;
      background: rgb(0, 255, 221);
      height: 100%;
      animation: changeW 3s infinite alternate;
    }

    @keyframes changeW {
      0% {
        width: 200px
      }

      100% {
        width: 500px;
      }
    }

    .content {
      overflow: hidden;
      height: 100%;
      background: rgb(238, 115, 8);
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="menu">我是菜单</div>
    <div class="content">我是内容</div>
  </div>
</body>
</html>